1. Field of the Invention
The present invention relates to a method for preparing microparticles containing an encapsulated catalyst for hydrosilylation reactions. More particularly, the present invention relates to a method for preparing these microparticles in a form that can be effectively used as a hydrosilylation-reaction catalyst because the catalytic activity of the microparticles is inhibited at temperatures below the softening point of the thermoplastic resin, and only becomes evident at temperatures at or above the softening point of the resin.
2. Background Information
Hydrosilylation is one of a limited number of reactions resulting in silicon-carbon bond formation. This reaction is therefore widely used for synthesizing organosilicon compounds and for crosslinking or curing silicone elastomer compositions.
A disadvantage of hydrosilylation is the ability of this reaction to occur at room temperature in the presence of extremely small amounts of a hydrosilylation catalyst. Maintaining hydrosilylation reaction mixtures in an unreacted state for extended periods of time at room temperature has proven to be an intractable problem and a particularly serious drawback for silicone elastomer compositions which cure by this reaction.
One method that has been used to overcome this disadvantage involves use of a hydrosilylation catalyst in the form of a powder made from a mixture of the catalyst and a thermoplastic resin.
For example, U.S. Pat. No. 4,481,341, which issued to Schlak et al on Nov. 6, 1984 describes preparation of a powdered hydrosilation catalyst composition with an inhibited catalytic activity for hydrosilylation reactions by grinding or pulverizing a mixture of a hydrosilylation catalyst and a thermoplastic resin.
Japanese published application no. 53/41,707 teaches a similar resin-encapsulated hydrosilylation catalyst where the resin is a silicone resin. Laid open Japanese application number 58/37053 proposes blending a powder containing a hydrosilylation catalyst and a silicone resin into curable organosiloxane elastomer compositions.
While the methods described in the aforementioned prior art are effective when applied to catalysts with relatively low activities, these methods have not been effective in inhibiting the catalyst activity of highly active hydrosilylation catalysts.
An additional disadvantage of silicone resin-encapsulated catalysts prepared by grinding and pulverization is the difficulty of obtaining a spherical morphology for the silicone resin powder. In particular, it is difficult to obtain spherical particles below about 10 micrometers in diameter. As a result of these problems, the silicone resin powder prepared by grinding and/or pulverization cannot be homogeneously dispersed in a curable composition. The use of this type of silicone resin powder as a curing catalyst for silicone elastomer compositions therefore has the fatal shortcoming of an incomplete crosslinking reaction.
Methods for solving the problems associated with resin-encapsulated catalysts prepared by grinding or pulverization are described in Laid Open Japanese Patent Applications 64/45468, 64/47442, 64/51140 and 02/4833. The teaching in these references can be summarized as two approaches. The first approach comprises preparing a solution of the hydrosilylation reaction catalyst and thermoplastic resin. The solvent selected should be able to dissolve the other two ingredients and is compatible with these ingredients. This solution is emulsified in an aqueous surfactant solution. The solvent is then removed by evaporation to yield a thermoplastic resin powder containing the hydrosilylation-reaction catalyst.
In accordance with the second approach taught by the prior art, a powdered resin containing a hydrosilylation catalyst is prepared according the first method and then washed with a solvent which can dissolve the hydrosilylation catalyst but not the thermoplastic resin. This produces a powder in which the catalyst has been removed from the surface and near-surface of the thermoplastic resin particles.
The first method can in fact produce a microparticulate powder with an average particle diameter of 10 micrometers or less, making possible a complete and thorough crosslinking reaction in silicone elastomer compositions. The second method inhibits catalytic activity even for highly active hydrosilylation catalysts. Resin encapsulated catalysts prepared using either of these two methods can be incorporated into curable compositions that can be stored for long periods of time at room temperature after all the ingredients have been mixed. The substantial time and labor required for the production of the desired powdered resin/catalyst mixture using these two methods increases the cost of implementing these methods on an industrial scale. Moreover, the microparticulate powder produced by these methods is prone to aggregation, which hinders its homogeneous dispersion throughout a curable organosiloxane composition.
The present method was developed as the result of extensive research by the present inventors directed to solving the problems associated with prior art resin-encapsulated hydrosilylation catalysts. One objective of the present invention is providing a highly productive method for preparing a resin-encapsulated hydrosilylation catalyst in the form of microparticles that can be efficiently used as a hydrosilylation-reaction catalyst. The activity of the catalyst is only evident at temperatures equal to or above the softening point of the thermoplastic resin.